(1) Field of the Invention
The invention relates to a rotary wing aircraft with a tail supporting a tail rotor, possibly at least one tail rotor being an electrically driven tail rotor.
(2) Description of Related Art
The dominant rotary wing aircraft configuration in the present time is based on helicopter basic design including a single main rotor and an auxiliary tail rotor to counter torque and provide directional yaw control. The tail rotor is mounted at the rear end of a supporting structure—the tail boom—behind the main fuselage. This tail boom is typically a single beam element.
A fin arranged at the same location as the tail rotor provides for directional stability during forward flight hence relieving the tail rotor and reducing the power needed for anti-torque. One or more horizontal tail planes are arranged as well at the aft portion of the tail boom or on top of the fin in order to provide for pitch stability. The large lever arm between the center of gravity and the tail rotor improves the efficiency of the active and passive stabilizing system and reduces the interaction with the main rotor.
Conventional tail boom constructions of helicopters are typically provided with a slim cross-section which allows for sufficient clearance for rear loading and for avoiding collisions with the blades of the main rotor. Slim cross-sections of a tail boom beam element lead to reduced bending and torsional stiffness and high root stresses at the mounting of the tail boom beam element to a fuselage of the helicopter, the bending stiffness being a potential concern in terms of the dynamic behavior of the helicopter and the root stresses being mainly a serious source of strength and fatigue issues.
Moreover, the root attachment of a centrally arranged tail boom is allocated on an upper part of the fuselage, which leads, on the one hand, to a reduction of cabin volume and loading clearance. On the other hand the root attachment sets important requirements on the front root structure of the tail boom in terms of fire resistance due to its integration within the engine deck area of the helicopter, leading to a higher structural complexity and the use of expensive materials and complex interface joints. Typical tail booms are impacted by the hot exhaust gases of the helicopter's engines, especially during hover, setting important requirements to the choice of structural materials in terms of their heat resistance which translates to less design flexibility and larger material costs.
The disassembly of a typical continuous circumferential joint of a single tail boom beam element with a large amount of fasteners from a fuselage or the integration of pivotable features between a single tail boom beam element and a fuselage is complex. The integration of pivotable features within conventional tail booms is inefficient since it does not allow large folding angles relative to a fuselage due to the central arrangement of the tail booms with cross-sections widths considerably smaller than the width of the fuselage. Integrated tail booms shaped as a smooth continuation of the fuselage hull lead to reductions in drag during forward flight but, on contrary, to an increase of down-load in the hover.
A streamlined tail boom may be practical on executive transports, but for other purposes rear loading ramps or clamshell doors may be needed which result in typical pod and boom designs with higher drag during forward flight. The integration of pivotable or dismountable streamlined tail booms lead moreover to a considerable structural weight increase and to less efficient folding characteristics.
A typical cylindrical shape of a tail boom is optimal for torsional stiffness but it is generally less efficient when subjected to a transverse flow. The unstable characteristics of a transverse airflow across a cylindrical shape are a possible source of shaking in the hover of the helicopter, which is a phenomenon directly linked to the bending stiffness of the tail boom.
The tail boom houses the transmission and controls of the tail rotor, antennae and other systems, i.e. on-board equipment (e.g. electric, mechanical, electronic, tactical, etc.). Transmission shafts are typically arranged outside and on top of the load carrying tail boom structure to allow for easy inspection and maintenance.
A wide variety of different approaches have been suggested in order to support or even fully replace the anti-torque duty of the tail rotor or just to reduce its noise generation, some of them using the down-wash airflow generated by the main rotor in order to passively generate an additional lift force to counter-torque. A tail boom structure has to be designed according to static, dynamic and fatigue requirements, hence showing a certain bending and torsional stiffness, an adequate strength and an appropriate mass.
Two types of tail boom are present, which differ from each other in the way of structural integration within the main fuselage body. The first typical tail boom is a single, slim beam element which is attached on its front end to the fuselage aft and top region. The typical cross-section of those tail booms is essentially cylindrical with a flat top or bottom base. The second type of tail boom is one single boom integrated within the fuselage body with a smoothly tapered transition from the central fuselage body to the tail. The cross section is however larger than that of the first design hence leading to increased down-loads generated by the down-wash of the main rotor.
Besides, twin boom configurations are seldom and have been especially suggested for high speed rotorcraft configurations, the twin booms being typically parallel and attached at their rear end to the tips of a transverse tail plane.
Some rotary wing aircrafts are designed with foldable capabilities so as to reduce their overall dimensions to facilitate stowage in confined available spaces. Typical folding capabilities are limited to the folding of the blades rearwardly and the folding of a rear portion of the tail forwardly.
Most rotary wing aircrafts in use carry tail rotors that are mechanically driven from the main turboshaft powerplant through a complex system of shafts, transmissions, and couplings. The configuration requires a direct mechanical power transmission from the powerplant to the tail rotor (deflections require heavy gear boxes) intensive maintenance and continuous inspection.
Illustrating the above, the following documents have been considered in the application, i.e.: GB2359533, GB2449743, U.S. Pat. Nos. 2,973,923, 6,050,521, 3,921,938, US2004/0031879, US2012/0280079, CA2316418, DE102011010097, DE202010014056, DE202012002493, DE102006004798, EP2690010, GB2320477, RU2246426 and US2009/0277991.
The document GB2359533 discloses a dismountable helicopter with a modular airframe and rotor unit. The torque compensating rotor in the tail of the helicopter is driven by a thin-walled drive shaft, detachably connected to the main power drive.
The document GB2449743 describes an aircraft which can be dismounted into different parts; inter alia its rear part is entirely separable from the front part of the aircraft.
The document U.S. Pat. No. 6,050,521 discloses a releasable coupling for a power transmission to a tail rotor of a foldable-tail-section helicopter. The coupling consists of two coupling assemblies, one associated with the front section of the helicopter, the other one associated with the tail section of the helicopter. The two coupling assemblies are coupled via radial toothings, cooperating telescopically with each other.
The document U.S. Pat. No. 3,921,938 A shows a typical cantilever tail boom but with an asymmetrical arrangement with respect to the aircraft's longitudinal axis. The tail boom is pivotable about its front attachment to the fuselage. This arrangement allows for folding capabilities and a minimum stowage volume.
The document U.S. Pat. No. 6,729,576 shows a special structural design for a typical cantilever tail boom arrangement using composite materials.
The document US2012/00280079 shows a special design of a typical cantilevered tail boom with a streamlined cross-section of the tail boom in order to generate an anti-torque force by the effect of the downwash from the rotor.
The document U.S. Pat. No. 2,973,923 describes a helicopter construction having a cabin and a main body that includes cabin framework as well as a tail for a counter torque rotor. The main body is formed of welded tubular metallic members, suitably cross braced. The main body is box shaped in cross-section, having no appreciable depth throughout. The main body has supports that carry a pair of motors for pivoting four rotor blades.
The document EP2690010 describes a compound helicopter with a pair of tail booms and a pair of fixed main wings and a pair of additional propulsive devices, for lift and thrust during forward cruise flight.
The document DE102011010097 describes a remote controlled model helicopter. Two receiver items are releasably connected to side plates of a helicopter frame. The helicopter frame is manufactured as an injection molded component. A tail boom mount is disposed at the level of a tail pulley so that a rear strap may be passed through a tail boom for a tail rotor. Under the tail boom, two struts support the tail boom and are attached to the helicopter frame.
The document DE202010014056 describes a radio controlled model helicopter. A rear fairing has side plates extending from the body to the tail of the model helicopter. The rear fairing has an open design for airflow through the cell and the tail boom in the direction of the rotor axis. The rear fairing has an integrated side plates fin. The rear fairing has for support of a stern tube, located fixings that give the side panels lift and shape. The rear fairing has a fastening on a chassis of the model helicopter to produce a self-supporting structure.